Since precision press-molded aspherical lenses and the like that are made with optical glass having broad dispersion characteristics (for example, an Abbé number, vd, of 17 to 72) are extremely useful in optical design, the demand for the optical glass used in such precision press molding is increasing. However, large quantities of lead oxide have been incorporated into the optical glass compositions of many of the glasses and the pressing temperature has been reduced to increase the service life of the pressing mold employed in precision press molding. For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 1-308843 discloses an optical glass for precision pressing comprised of (denoted as weight percentages): SiO2: 15 to 50 percent, PbO: 30 to 58 percent, Li2O: 0.1 to 7 percent, Na2O: 0 to 15 percent, K2O: 0 to 15 percent where Li2O+Na2O+K2O: 3 to 25 percent; La2O3: 0 to 15 percent, MgO: 0 to 10 percent, TiO2: 0 to 10 percent, where La2O3+MgO+TiO2: 0.1 to 20 percent, ZrO2: 0 to 5 percent, Al2O3: 0 to 10 percent, where ZrO2+Al2O3: 0.1 to 10 percent; ZnO: 0 to 20 percent, B2O3: 0 to 15 percent, Y2O3: 0 to 5 percent, Gd2O3: 0 to 5 percent, CaO: 0 to 10 percent, SrO: 0 to 15 percent, BaO: 0 to 9 percent; Nb2O5: 0 to 15 percent, Ta2O5: 0 to 5 percent, WO3: 0 to 5 percent, P205: 0 to 5 percent, As2O3: 0 to 1 percent, Sb2O3: 0 to 5 percent.
However, precision press molding is normally conducted in a nonreactive or weakly reducing atmosphere to prevent oxidation of the pressing mold. When precision press molding the above-described glass containing a large amount of lead oxide among the glass components, lead oxide at the surface of the glass is reduced, precipitating out as metallic lead on the surface of the lens. This then adheres to the mold material used to mold precision press-molded lenses, compromising the precision of the transfer surface of lenses that are precision press molded and requiring maintenance to remove the metallic lead adhering to the mold, which is not suited to mass production. Further, the melting of the glasses containing large quantities of lead oxide disclosed in the above-cited patent publication presents major problems in terms of environmental pollution. Thus, the glass disclosed in above-cited patent publication is unsuited to precision press molding.
The present invention, devised to solve the above-stated problems, has for its object to provide preforms for precision press molding comprised of optical glass imparting broad dispersion characteristics, permitting a low pressing temperature suited to precision press molding, and affording high stability; a method of manufacturing such preforms; optical elements obtained by precision press molding this preform; and a method of manufacturing such optical elements.